Integrated optical devices provide low cost and compact alternative to systems based on bulk optics such as lenses, filters and mirrors. An important element particularly useful in optical communications is a channel analyzer. This element is based on a filter, which can either he timed across a given bandwidth, or fixed at a predetermined frequency. In both cases, the filtered output is detected and analyzed to obtain the frequency information.
In most monitoring applications, there is a need to minimize the loss of the monitored signal caused by the monitoring function. This is typically achieved by directing a small portion of the guided light to tie optical filter. This approach is disclosed in U.S. Pat. Nos. 5,323,409, 6,385,217, 6,134,253 and 6,289,028; and is generally illustrated in FIG. 1. As shown, guided light (e.g. light generated by a laser source) propagating in a waveguide 101 interacts with a tap accommodated at a region 102 of the waveguide, and a small portion of light is directed to a spectral monitor 103 while the other portion of the guided light remains in the waveguide 101 and propagates through its successive segment 104. The loss is thus determined by the amount of light directed out of the guide (i.e., the ratio between the portion of the optical signal directed out of the guide into the filter and the initial amount of optical signal within tie guide). Minimization of loss can be obtained mainly by reducing the portion of the optical signal to be detected.
A spectral monitor can utilize resonators and micro resonators. This technique is disclosed in WO 01/81962 assigned to the assignee of the present application. Here, a spectral analysis filter is composed of two ring-base compound resonators connected in parallel through a common linear waveguide and serves as a compound high Q optical ring resonator structure. The output linear waveguide of the structure is connected to a detector. The Q of filter is determined by the coupling factor describing the amount of light that is coupled into the filter at every round trip, and is also determined by the optical losses in the cavity and the ring radius. The filter structure is connected to an optical network (link) via a tap coupler, which taps a small amount of light. The compound resonator is disclosed in WO 01/27692 assigned to then assignee of the present application, and presents a combination of two spaced-apart waveguides and at least two spaced-apart resonator-cavity loops accommodated between the two waveguides and connected to each other through sections of the waveguides, such that the spaced-apart resonator-cavity loops and the waveguide sections create a closed loop compound resonator for storing optical energy of a predetermined frequency range. The physical characteristics of compound resonator are controllable (via application of an external field, each heating) to adjust the optical storage characteristics of the compound resonator.